Babies are born with the instinct to suckle milk from their mothers' breasts, but it is often necessary for them to drink liquids from other sources. Babies are unable to drink liquids from glasses or cups without spilling so it is common throughout the world to feed liquids to babies in nursing bottles, also known as baby bottles. A nursing bottle features a rubber nipple with a small hole in its tip secured across an opening in the top of a liquid container. A nursing bottle is used by filling the container with liquid, securing the nipple, inverting the bottle, and placing the nipple into the baby's mouth. The baby then sucks on the nipple to withdraw the liquid.
A typical nursing bottle is tightly sealed except for the small opening in the nipple. As the baby nurses, the liquid volume inside the bottle decreases and the air volume increases. However, ambient air is unable to enter the bottle so a partial vacuum is created. The partial vacuum, in turn, impedes the flow of liquid out the nipple and forces the baby to suck harder to withdraw the liquid. As the baby sucks harder on the nipple, ambient air inadvertently and inevitably enters the baby's mouth and stomach. Excessive air in the stomach and other parts of the alimentary canal causes colic, a condition characterized by abdominal discomfort and pain. See generally O. P. Mathew, Science of Bottle Feeding, The Journal of Pediatrics, October 1991, 511; and W. R. Treem, Infant Colic, Pediatric Clinics of North America, October 1994, 1121.
Many attempts have been made to provide a nursing bottle with an air vent to enable ambient air to enter the container during use. For example, Rodetick, U.S. Pat. No. 598,231, issued Feb. 1, 1898, discloses a nursing bottle having a U-shaped air tube. One end of the tube communicates with the top of the container interior while the other end communicates with the ambient air outside the bottle. When the bottle is inverted, liquid rises into the tube and impedes the flow of air into the interior of the container. If the bottle is placed upright quickly, the liquid in the tube does not have a chance to drain and it remains in the tube. When the bottle is again inverted, the liquid spills out the end of the tube which communicates with the ambient air. Other nursing bottles with air vents are disclosed in Van Cleave, U.S. Pat. No. 927,013, issued Jul. 6, 1909; Davenport, U.S. Pat. No. 1,441,623, issued Jan. 9, 1923; and Perry, U.S. Pat. No. 2,061,477, issued Nov. 17, 1936. None of these nursing bottles completely solves the problem of maintaining the interior of the bottle at atmospheric pressure while preventing leaks and spills. Accordingly, a demand still exists for a nursing bottle which prevents the formation of a partial vacuum inside the bottle during nursing and yet does not result in spills.